This invention relates to a method of and apparatus for enhancing the performance of MS/MS mass spectrometers that involve two sequential mass analyzing steps. This invention more particularly relates to such a technique effective in a mass spectrometer with axial ejection from a linear ion trap with axial ejection.
It is common in mass spectrometry to use at least two mass spectrometers in series separated by a gas filled collision cell. In triple quadrupole instruments the first mass spectrometer, often designated as MS1, is a resolving quadrupole followed by a collision cell operated in total ion mode and finally a second mass resolving quadrupole, often designated as MS2. The collision cell, in known manner includes another quadrupole rod set. These quadrupole rod sets are commonly referred to as Q1, Q2 and Q3 respectively and the ion path is often referred to as QqQ, where Q denotes a quadrupole rod set that can be operated in a mass resolving mode, and q a rod set used for collision induced dissociation and fragmentation. Such a configuration will often include a further upstream rod set, commonly denoted Q0, which is operated just as an ion guide. It serves to focus the ions and further eliminate gas from the ion stream, usually generated by an atmospheric source.
MS/MS experiments, as they are usually known, can be carried out in such instruments and involve choosing specific precursor ions with Q1, fragmenting the precursor ions in a pressurized Q2 via collisions with neutral gas molecules to produce fragment or product ions, and mass resolving the product ions with Q3. This technique has proven to be very valuable for identifying compounds in complex mixtures and in determining structures of unknown substances. Several possible scanning modes of MS/MS operation are well known and these are:
(1) setting MS1 (Q1) at a particular precursor ion m/z value to transmit a small range of mass resolved ions into the collision cell (Q2), while (Q3) is scanned to provide a product ion spectrum;
(2) setting MS2 (Q3) at a particular product ion m/z value and then scanning MS1 (Q1) to provide a precursor ion spectrum; and
(3) scanning both MS1 (Q1) and MS2 (Q3) simultaneously with a fixed m/z difference between them, to provide a neutral loss spectrum.
Thus the m/z value of a precursor ion, a product ion, or an ion generating a given neutral fragment ion can be determined using MS/MS techniques.
MS/MS techniques generally provide better detection limits than a single stage of mass analysis due to the reduction of chemical noise which is the signal due to generation of ions from other components within the sample, the solute, or the environment surrounding the ion source or within the mass spectrometer itself. MS/MS reduces this nonspecific ion signal and results in better signal-to-noise even though there are two stages of mass resolution which reduce the total number of ions at the detector.
MS/MS instruments based on scanning mass spectrometers, such as quadrupoles, reject the majority of ions formed at any given time within the scan cycle; the essence of scanning is to select a narrow m/z range for further analysis and reject all other ions. Thus, these instruments have inherently poor duty cycles.
Triple quadrupole mass spectrometers are often referred to as xe2x80x9ctandem in spacexe2x80x9d devices since the precursor ion isolation, fragmentation, and fragment ion mass resolution are effected with different ion optical elements located at physically different locations in the ion path. Ion trap mass spectrometers have potentially much greater duty cycles than such tandem in space quadrupole mass spectrometers since all of the ions within the mass spectrometer can be scanned out and detected. The origin of this duty cycle enhancement arises from the fact that ion trap mass spectrometers are typically filled with a short pulse (typically 5-25 ms) of ions from which a complete mass spectrum is generated. On the other hand, in the time required to fill and scan an ion trap, a conventional beam type or tandem is space quadrupole mass spectrometer can only acquire mass spectral information over a very small mass range.
Hybrid MS/MS instruments such as QqTOF instruments, in which the final stage of mass analysis (MS2) is accomplished via a non-scanning time of flight (TOF) mass spectrometer have a duty cycle advantage over QqQ instruments in that the TOF section is not a scanning mass spectrometer, and all of the ions in the product ion mode are collected within a few hundred microseconds. These instruments are typically 10-100 times more sensitive than conventional QqQ instruments in the product ion scan mode of operation.
However in the precursor ion or neutral loss scan modes, in which Q1 is scanned and the ion signal of a particular product ion is measured, the problem of the low duty cycle of a scanning mass spectrometer reappears. In other words, while the TOF section can indeed measure ions over a wide range, in these experiments, one is only interested in an ion of particular m/z value. Additionally, there is an inherent incompatibility between quadrupole stages, which operate in a continuous flow mode, and a TOF stage with intermittent or pulsed operation. For the QqTOF instruments, the overall ion path transmission is considerably less than that of a QqQ instrument (typically xcx9c1% as efficient as a QqQ due largely to this incompatibility). This is exacerbated by the low duty cycle that reappears in the precursor ion and neutral loss scan modes. Consequently many TOF scans must be acquired at each parent ion mass to generate a precursor ion scan with reasonable signal-to-noise and this also applies for the neutral loss scan. This can increase the time acquired for each such experiment to tens of minutes.
In applicant""s U.S. Pat. No. 6,177,668 and also in published international application WO 97/47025, there is disclosed a multipole mass spectrometer provided with an ion trap and an axial ejection technique from the ion trap. The contents of these applications are hereby incorporated by reference.
The technique relies upon emitting ions into the entrance of a rod set, for example a quadrupole rod set, and trapping the ions at the far end by producing a barrier field at an exit member. An RF field is applied to the rods, at least adjacent to the barrier member. The barrier member is supplied with a barrier field to trap ions, and the barrier and RF fields interact in an extraction region adjacent to the exit end of the rod set and the barrier member, to produce a fringing field. Ions in the extraction region are energized, to eject, mass selectively, at least some ions of a selected mass-to-charge ratio axially from the rod set and past the barrier field. The ejected ions can then be detected. Various techniques are taught for ejecting the ions axially, namely scanning the frequency of an auxiliary AC field applied to the end lens or barrier, scanning the amplitude of an RF voltage applied to the rod set while applying a fixed frequency auxiliary voltage to the end barrier and applying an auxiliary AC voltage to the rod set (again scanned in frequency) in addition to, or instead of, that on the lens and the RF on the rods.
It has now been realized that this technique can be used to enhance the performance of a triple quadrupole or QqTOF instrument, or indeed in general any tandem in space MS/MS instrument including a collision cell between two mass analyzers.
In accordance with a first aspect of the present invention, there is provided a method of mass analyzing a stream of ions, the method comprising the steps of:
(1) passing the ions through a first mass analyzer to select a precursor ion;
(2) subsequently passing the precursor ions into a collision cell containing a gas, to cause dissociation of the precursor ions and the formation of fragment ions, for subsequent analysis, wherein the method includes trapping the fragment ions in the collision cell by means of a potential barrier, and scanning the fragment ions axially out therefrom by excitation of the ions, whereby the fragment ions can traverse the potential barrier.
Preferably, the method includes providing a barrier at an exit from the collision cell and providing a quadrupole rod set in the collision cell, the method comprising scanning the ions out of the collision cell by applying at least one of the following group of signals: An AC signal to the barrier; an AC signal to the rod set; and an RF signal to the rod set, wherein the method includes scanning ions out of the quadrupole rod set by at least one of:
(a) scanning the amplitude of the RF signal;
(b) scanning the frequency of the AC signal; and
(c) scanning the amplitude of the RF signal, without any applied signal, to effect ejection of ions approaching a q-value of approximately 0.9.
Ions exiting from the collision cell can be detected with a detector or with a mass spectrometer, more preferably a time of flight mass spectrometer. The time of flight mass spectrometer is advantageously arranged orthogonally to the collision cell.
The ions can be pre-trapped in a first quadrupole rod set upstream of the first mass analyzer, so that the ions can then be admitted as pulses into the first mass analyzer. Then, a further quadrupole rod set can be provided as the first mass analyzer, for selecting the precursor ions.
The method of the present invention can include effecting a precursor scan by scanning the fragment ions out of the collision cell and detecting a selected ion or ions and stepping the first mass analyzer through a range of mass-to-charge ratios to select a range of precursor ions for recording against the selected ion or ions detected.
Alternatively, the method can be used to effect a neutral loss scan, the method comprising selecting a precursor ion in the first mass analyzer having a first mass-to-charge ratio and detecting fragment ions having a second mass-to-charge ratio leaving the collision cell, wherein the method comprises maintaining a fixed neutral mass difference between the first and second mass-to-charge ratios and stepping the first and second mass-to-charge ratios through desired ranges.
Another aspect of the present invention provides an apparatus, for mass analyzing a stream of ions, the apparatus comprising: a mass analyzer; a collision cell; a means of trapping ions in the collision cell; a means for exciting ions to enable ions to be scanned out of the collision cell axially; and a time of flight mass spectrometer for receiving ions from the collision cell.
Preferably, the collision cell includes a quadrupole rod set and a barrier providing an interquad aperture between the quadrupole rod set and the time of flight mass spectrometer, and voltage supply means connected to the quadrupole rod set and the barrier, for supplying at least one of: an AC signal to the barrier; an AC signal to the rod set; and an RF signal to the rod set, and wherein the apparatus includes a chamber in which the quadrupole rod set is mounted and means for supplying a collision gas to the chamber.
More preferably, the first mass analyzer comprises a quadrupole rod set mounted axially upstream from the collision cell, and the apparatus further including voltage supply means for supplying RF and resolving DC voltages to the quadrupole rod set of the first mass analyzer.
The apparatus can include a further quadrupole rod set, axially aligned with the quadrupole rod set of the collision cell and the quadrupole rod set of the first mass analyzer and provided upstream of the first mass analyzer, and wherein the apparatus also includes a plate providing a further interquad aperture between the further quadrupole rod set and the mass analyzer, whereby ions can be pre-trapped in the further quadrupole rod set.
Preferably, the time of flight mass spectrometer comprises an orthogonal time of flight mass spectrometer. Moreover, the time of flight mass spectrometer can include a straight through detector, whereby to detect ions of a particular mass-to-charge scanned out of the collision cell, ions can be detected continuously at the detector without pulsed operation of the time of flight mass spectrometer.